fixed isWithinBar() crash when there were less than 2 items in pathSe…

[panthony]

Rebased version of #1550

I'm using c3js within a webapp based on Ember and it can happen that charts are drawn &
destroyed even before they finish loading.

When navigating I can often have the following error happening:

[codecov-io]

Codecov Report

Merging #2652 into master will decrease coverage by 0.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2652      +/-   ##
==========================================
- Coverage   80.14%   80.13%   -0.02%     
==========================================
  Files          57       57              
  Lines        4518     4520       +2     
==========================================
+ Hits         3621     3622       +1     
- Misses        897      898       +1

Impacted Files	Coverage Δ
src/shape.bar.js	98.64% <50%> (-1.36%)	⬇️

---

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[codecov-io]

Codecov Report

Merging #2652 into master will decrease coverage by 0.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2652      +/-   ##
==========================================
- Coverage   80.14%   80.13%   -0.02%     
==========================================
  Files          57       57              
  Lines        4518     4520       +2     
==========================================
+ Hits         3621     3622       +1     
- Misses        897      898       +1

Impacted Files	Coverage Δ
src/shape.bar.js	98.64% <50%> (-1.36%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov-io]

Codecov Report

Merging #2652 into master will decrease coverage by 0.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2652      +/-   ##
==========================================
- Coverage   80.14%   80.13%   -0.02%     
==========================================
  Files          57       57              
  Lines        4518     4520       +2     
==========================================
+ Hits         3621     3622       +1     
- Misses        897      898       +1

Impacted Files	Coverage Δ
src/shape.bar.js	98.64% <50%> (-1.36%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4643f4f...033a2e2. Read the comment docs.

[codecov-io]

Codecov Report

Merging #2652 into master will decrease coverage by 0.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2652      +/-   ##
==========================================
- Coverage   80.14%   80.13%   -0.02%     
==========================================
  Files          57       57              
  Lines        4518     4520       +2     
==========================================
+ Hits         3621     3622       +1     
- Misses        897      898       +1

Impacted Files	Coverage Δ
src/shape.bar.js	98.64% <50%> (-1.36%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4643f4f...033a2e2. Read the comment docs.

[kt3k]

@panthony LGTM. Thanks!